Personally I REALLY hope that we someone start on a tram. Maybe in Black Mesa as a small minor flashback that will have greater significance in the future of the game? I'd love to start the game all excited and hear, "Welcome to the Black Mesa Transit System..."

Has to start on anything moving. HL started on a tram(Blue Shift also, Opposing Force on an chopper), HL2 on a train, HL2:EP1 was an exception but whole game was about getting to the train, so legit, HL2:EP2 started on a devastated train. So they either have to do something to get us on a train, or bundle Episode 3 that will consist only of Gordon walking to the helicopter.